The present invention relates to a method and apparatus for mixing low-frequency signals received in sampled digital form in different time slots of the same time-division telephone link for onward transmission to a set.
Such a method and apparatus make it possible for a single telephone receiver unit (such as an earphone or a loudspeaker) to receive signals simultaneously from several distinct signal transmitting units. For example, a tone may thus be transmitted to a telephone set which is already in use, or an operator may intervene as a third party in a call, or a single telephone may receive audio or other signals from a plurality of other telephones in a conference call.
In space-switched telephone systems where telephone audio is transmitted in the form of analog electrical signals, signals from two different telephone transmitter units (and in particular from two different telephone sets) are superposed by making Y-connections in the wires carrying the audio signals. Unfortunately this makes it difficult to satisfactorily set up a conference call between a plurality of time-switched telephone sets.
In present time-switched telephone systems, the signals generated by electronic or human sources are produced in digital form or converted thereto at telephone transmitter units, which integrate the signals into the time-switched network in which transmission takes place via physical transmission links (herein called time-division links) whose time of use is divided into frames of identical structure. These frames are subdivided into an equal number of time intervals each of which corresponds to a time slot which can be reserved for a telephone transmitter unit for the duration of the message which it is to transmit. Consequently, each telephone transmitter unit produces or converts the data to be transmitted into a succession of digital samples intended to be transmitted in a specific time slot.
Conventionally, the transmitted samples are compressed in accordance with well-defined laws so as to reduce the number of bits that need to be transmitted for reconstituting the telephone signal at a receiver unit. The disadvantage of this compression technique is that it results in the sum of two samples not itself being a decodable sample. This complicates putting a plurality of telephone transmitter units into simultaneous communication with the same receiver unit.
While it is easy to transmit signals coming from a single transmitter unit to a plurality of receiver units by duplicating each sample, it is considerably more complicated by collect together a plurality of samples coming from different telephone transmitter units for simultaneous onward transmission in the form of a single decodable sample. This has conventionally required the samples to be individually decompressed so that they can be combined by simply adding them together, followed by recompression of the sum signal to obtain a transmittable sample.